Rework find dialog (part one)

This is work in progress but I don’t think any functionality is missing compared to previous commit, although some functionality might be less polished, e.g. the action buttons don’t properly enable/disable and the height of the results list gets lost when hiding it.

Some of the stuff that has changed / improved:

 * The find/replace text fields adjust their height to encompass the content (closes #94). Presently though the initial height of the ontrols is one line regardless of content (but they should adjust on first edit).
 * You can use Save All (⌥⌘S) to save affected files after Replace All (closes #558).
 * The key equivalents / actions available in the Find dialog is now easier to find via the action pop-up, which also has enables accessibility.
 * Using Next (⌘G) or Previous (⇧⌘G) with search results will move selection up/down.
 * Using Find All with ‘in’ set to ‘Selection’ will find and select all matches in the current document, which fixes #425, though it might be more desirable to show the results in the find dialog (like Find All does for a document or folder).
ARC: Update Find framework